Hydrogen Fuel Cells

Julian Kates-Harbeck
October 24, 2010

Introduction

Individual transportation is a topic that is very
important to most people. It is an important part of personal freedom
and as such deeply engrained in the culture of most developed countries.
Currently, the vast majority of cars are powered by internal combustion
engines (ICEs) that run on either diesel fuel or gasoline. Both of these
fuels are hydrocarbons and were produced by refining crude oil.

There are good reasons for why gasoline and diesel
are used so widely. First, they have very high energy densities. In
fact, the energy density per unit volume of gasoline and diesel is
higher than that of any other currently available chemical fuel. [1]
Second, they are liquids at standard temperatures and pressures which
makes it easy to store, transport and distribute them. Third, they can
currently be produced rather cheaply from crude oil.

Unfortunately, there are several fundamental problems
with our current use of these fuels. The oil reserves of our planet are
finite, and most scientists agree that the reserves will be depleted
sometime within the next two centuries. Furthermore, the combustion of
fossil fuels like gasoline and diesel produces CO2, one of
the main contributors to the greenhouse effect and the resulting climate
change. Evidently, whether it is for the sake of the environment or
simply because the reserves are gone, we will have to switch from fossil
fuels to alternative fuels within the next centuries, or even within the
next decades.

Evidently, hydrogen fuel cell technology is a
promising candidate for the future of powering individual
transportation.

Hydrogen Infrastructure

Hydrogen can be produced easily from water using
electrolysis. Ideally, the electricity used for this process will in the
future come from renewable energy sources. If we assume so, then the
hydrogen fuel cycle is inherently clean and emission-free. Since
hydrogen is a very light and volatile substance, efficient storage is
currently one of the greatest challenges for a prospective hydrogen fuel
cell technology in automobiles. In order to reach reasonably high
densities for pure hydrogen, it must either be stored under high
pressures or in liquid form. This means that the tank in question will
either have to withstand several hundred atm. of pressure or be equipped
with a sophisticated cryogenic system, which in any case poses a
significant engineering challenge when considering automobiles. [2]
There also exist more advanced technologies to store hydrogen; in metal
hydride storage for example the hydrogen is kept within the metal
lattice of some carrier material, thereby more densely packing the
single hydrogen molecules. Current research makes use of the same basic
idea, but uses carbon nanotubes instead of metals. Hydrogen will be
distributed similarly to the way gasoline and diesel are distributed
nowadays. Car owners will refuel at the hydrogen equivalent of gas
stations by plugging a nozzle into their car. It remains questionable as
to how much of the currently existing infrastructure can be reused and
how much will have to be built anew.

Description of Hydrogen Fuel Cells

The basic chemical reaction driving an ICE and a fuel
cell is the same. In an ICE, the combustion of the fuel is used to drive
a heat engine. By the laws of thermodynamics such heat engines have
inevitable losses and can only reach efficiencies below certain limits.
In practice, gasoline engines reach efficiencies of about 20-25%, while
diesel engines are slightly higher - about 40-45%. [2] In fuel cells,
the electron transfer of the chemical reaction is used directly to drive
an external circuit in which useful electric work is performed. The
process is kept reversible, which eliminates the basic thermodynamic
limitations that internal combustion engines exhibit. The efficiencies
of fuel cells are therefore substantially higher than those of ICEs.
Fuel cell efficiencies can reach up to 60%. [2]

In a hydrogen fuel cell, the fuel is molecular
hydrogen (H2), which is combined with oxygen O2
from surrounding air to produce water (H2O). The hydrogen is
inserted on one side of a so-called PEM (proton exchange membrane) which
permits the flow of protons through it but doesn't allow the flow of
electrons. Using catalysts, the hydrogen as well as the oxygen molecules
are broken up into single atoms at the electrodes. In order to recombine
with oxygen atoms on the other side of the membrane to form water
molecules, the proton separates from its electron and moves through the
membrane while the electron is guided through an external circuit where
it performs useful work.

Advantages and Disadvantages

Currently, there are only very few sustainable
alternatives to fossil fuels that seem to have the potential to replace
them with all their functionality and convenience. First, there are
battery powered electric vehicles (EVs). EVs are already commercially
available which suggests that they are a somewhat viable alternative to
fossil fuels. On the other hand, there are some major disadvantages. For
example, batteries have a very low energy density compared to liquid
fuels. The battery packs in currently available EVs weigh several
hundred kilograms but only provide a range of about 200 miles. The
amount of hydrogen required to provide for such a range, even if
considering the weight of the storage tank, would be less. [2,3]
Additionally, the charging time is currently considerably longer than
the refueling time of a gas or diesel tank. While the full recharging
time of an EV is on the range of hours, a refueling stop for hydrogen
would work very similarly to the way current gas or diesel tanks are
refueled, which would make it a matter of minutes.

Apart from batteries and hydrogen fuel cells,
biofuels are another alternative to gasoline and diesel. Chemically,
biofuels are very similar to diesel fuel made from oil, which means that
they have almost equally high volume energy densities; higher than those
of batteries or hydrogen. On the other hand, the combustion reaction of
biofuels with oxygen produces the unwanted CO2.
Theoretically, this CO2 is in a closed cycle, since the
amount of CO2 released in the combustion should equal the
amount of CO2 captured during the growth of the biomass from
which the biofuel was made of. Currently though, the production process
involves many inefficiencies and therefore there are net CO2
emissions. Furthermore, the amount of agricultural land that would be
required to produce enough biofuels for the entire US car fleet is a
substantial fraction of the current US farmland and it is questionable
how much land the economy can afford to devote to biofuel
production.

Conclusion

Hydrogen fuel cells are a promising alternative to
current automobile fuels. They essentially combine the energy density
and the convenience of liquid fuels with the clean and efficient
operation of electric vehicles. Although certain aspects of the
technology such as efficient on-board storage still require some
improvement, there are no reasons why hydrogen couldn’t become an
equally convenient and attractive transportation fuel as diesel or
gasoline are today.